Inkjet printing mechanisms use pens which shoot drops of liquid colorant, referred to generally herein as "ink," onto a page. Each pen has a printhead formed with very small nozzles through which the ink drops are fired. To print an image, the printhead is propelled back and forth across the page, shooting drops of ink in a desired pattern as it moves. The particular ink ejection mechanism within the printhead may take on a variety of different forms known to those skilled in the art, such as those using piezo-electric or thermal printhead technology. For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481, both assigned to the present assignee, Hewlett-Packard Company. In a thermal system, a barrier layer containing ink channels and vaporization chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as resistors, which are energized to heat ink within the vaporization chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energized resistor. By selectively energizing the resistors as the printhead moves across the page, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text).
To clean and protect the printhead, typically a "service station" mechanism is mounted within the printer chassis so the printhead can be moved over the station for maintenance. For storage, or during non-printing periods, the service stations usually include a capping system which hermetically seals the printhead nozzles from contaminants and drying. To facilitate priming, some printers have priming caps that are connected to a pumping unit to draw a vacuum on the printhead. During operation, partial occlusions or clogs in the printhead are periodically cleared by firing a number of drops of ink through each of the nozzles in a clearing or purging process known as "spitting." The waste ink is collected at a spitting reservoir portion of the service station, known as a "spittoon." After spitting, uncapping, or occasionally during printing, most service stations have a flexible wiper, or a more rigid spring-loaded wiper, that wipes the printhead surface to remove ink residue, as well as any paper dust or other debris that has collected on the printhead.
To improve the clarity and contrast of the printed image, recent research has focused on improving the ink itself. To provide quicker, more waterfast printing with darker blacks and more vivid colors, pigment based inks have been developed. These pigment based inks have a higher solids content than the earlier dye-based inks, which results in a higher optical density for the new inks. Both types of ink dry quickly, which allows inkjet printing mechanisms to use plain paper. Unfortunately, the combination of small nozzles and quick-drying ink leaves the printheads susceptible to clogging, not only from dried ink and minute dust particles or paper fibers, but also from the solids within the new inks themselves. Partially or completely blocked nozzles can lead to either missing or misdirected drops on the print media, either of which degrades the print quality. Thus, keeping the nozzle face plate clean becomes even more important when using pigment based inks, because they tend to accumulate more debris than the earlier dye based inks.
Indeed, keeping the nozzle face plate clean for cartridges using pigment based inks has proven quite challenging. These pigment based inks require a higher wiping force than that previously needed for dye based inks. Yet, there is an upper limit to the wiping force because excessive forces may damage the orifice plate, particularly around the nozzle openings. Thus, a delicate balance is required in wiper design to adequately clean the orifice plate to maintain print quality, while avoiding damage to the nozzle plate itself.
Many previous wiping solutions used a cantilever wiping approach. In cantilever wiping, a flexible, low durometer elastomeric blade is supported at its base by a sled. While the sled may be stationary, in many designs it was moveable so the sled could travel to a position where the wipers engage the nozzle plate. Wiping was accomplished through relative motion of the wipers with respect to the nozzle plate, by either moving the wiper relative to a stationary nozzle plate, or by moving the nozzle plate relative to a stationary wiper. The earlier wiper positioning mechanisms included sled and ramp systems, rack and pinion gear systems, and rotary systems.
The flexibility of the cantilever wiper accommodates for variations in the distance between the nozzle plate and sled, also referred to as variations in the "interference" between the wiper and nozzle plate. That is, for a closer sled-to-nozzle spacing (or a "greater interference"), the wiper flexed more than it would for a larger spacing. The force transmitted to the face plate was determined by the degree of bending of the wiper blade, as well as by the stiffness of the wiper blade material. The stiffness of the wiper blade is a function of the geometry of the blade and of the material selected. For instance, one common measure of elastomeric flexibility (tested using a sample of a standard size) is known as the "durometer," including a variety of scales known to those skilled in the art, such as the Shore A durometer scale.
Besides focusing on the material selection for inkjet wipers, other research has investigated changing the contour of the profile of the wiper tip which contacts the printhead orifice plate. A revolutionary rotary, orthogonal wiping scheme was first used in the Hewlett-Packard Company's DeskJet.RTM. 850C color inkjet printer, where the wipers ran along the length of the linear arrays, wicking ink from one nozzle to the next to act as a solvent to break down ink residue accumulated on the nozzle plate. This product used a dual wiper blade system, with the wiper blades each having an outboard rounded edge and an inboard angular wiping edge. The rounded edges encountered the nozzles first and formed a capillary channel between the blade and the orifice plate to wick ink from the nozzles as the wipers moved orthogonally along the length of the nozzle arrays. The wicked ink was pulled by the rounded edge of the leading wiper blade to the next nozzle in the array, where it acts as a solvent to dissolve dried ink residue accumulated on the printhead face plate. The angular edge of the trailing wiper blade then scraped the dissolved residue from the orifice plate. The black ink wiper had notches cut in the tip which served as escape passageways for balled-up ink residue to be moved away from the nozzle arrays during the wiping stroke. One example of this system is described in greater detail in U.S. Pat. No. 5,614,930, assigned to the Hewlett-Packard Company. While the wiper tip had a rounded wiping edge and an opposing angular wiping edge, the remainder of the blade had a uniform rectangular cross sectional shape.
Another wiping system using a spring-loaded, non-bending upright wiper was first sold in the Hewlett-Packard Company's DeskJet.RTM. 660C color inkjet printer. Through a rocking action of the wiper blade and compression of the spring, manufacturing tolerance variations were accommodate for, including component variations in the service station, the printhead carriage, and in the pens themselves. Selection of the spring determined the perpendicular wiping force applied to the orifice plate. The wiper tip in this system had a triangular profile, with the remainder of the blade having uniform rectangular cross sectional shape. One example of this system is described in greater detail in U.S. Pat. No. 5,745,133, assigned to the Hewlett-Packard Company.
The wiping of a pen orifice plate or face has long been used to keep the face clean from crusted ink and other debris. However, over time it has been found that applying excessive forces perpendicular to the pen face may cause damage to the nozzle openings or orifices used to eject the ink from the printhead. FIG. 10 is a graph of the relatively constant level of perpendicular force, FP(X), applied across the entire width X of a printhead orifice plate WO when wiping with a prior art wiper blade having a rectangular cross sectional shape, with the force experienced by nozzle orifice openings being indicated at WN.
One set of solutions to this problem investigated changing the geometry of the wiper blade to lower this perpendicular wiping force. The two primary approaches investigated were (1) lengthening of the wiper blade to make it project further from its support toward the printhead, and (2) making the wiper blade thinner in the wiping direction. Unfortunately, both of these proposed geometric changes to the wiper blade were expected to result in a wiper which is more difficult to mold because the mold cavity would be either deeper or thinner, making it difficult to fill and resulting in higher scrap out rates from blades which have corners missing or voids formed therein.
Thus, the need exists for an improved wiper blade which adequately cleans crusted ink and other debris from an inkjet printhead, without applying excessive perpendicular force to the printhead in the area of the ink ejecting nozzle orifices.